1. Field of the Invention
The present invention relates to a defect inspection apparatus and a defect inspection method for inspecting a pattern defect of an object, and more particularly, the present invention relates to a defect inspection apparatus and a defect inspection method for inspecting a lithography mask for use in fabricating a semiconductor device or a liquid crystal display (LCD), or a semiconductor substrate or a liquid crystal substrate comprising a very small pattern.
2. Description of the Related Art
An improvement in yield is indispensable because manufacture of a large-scale integration (LSI) is generally costly. One of the causes which lower the yield includes a pattern defect of a lithography mask used when pattern is transferred onto a semiconductor wafer by lithography. In recent years, the minimum dimensions of a defect which must be detected have been smaller with the smaller LSI pattern dimensions formed on a semiconductor wafer. Thus, there is a need for providing a high precision defect inspection apparatus for inspecting a defect of a lithography mask for use in LSI manufacturing.
on the other hand, with the advancement of information technology or multimedia technology, as for the LCD, a liquid crystal substrate has been large-sized and a finer pattern of a thin film transistor or the like has been formed on the liquid crystal substrate. Therefore, it is required to inspect a very small pattern defect in a wide range. Accordingly, there is an urgent need for development of a defect inspection apparatus capable of inspecting a defect of a photo mask used when fabricating such a pattern of such a large area LCD and a large area LCD.
In a conventional defect inspection apparatus, a pattern formed on a sample under inspection such as a lithography mask is acquired as an image at a predetermined magnification by using an optical system similar to a microscope, and the acquired image is compared with design data, thereby carrying out inspection. The conventional defect inspection apparatus comprises a stage, a light source, a illumination optical system, a magnification optical system, a photoelectric converter section, a comparator circuit, a data memory, and a reference data generator section.
The sample is placed on the stage, and the stage moves, whereby a luminous flux scans on the sample. The light passed through the sample incomes into the photoelectric converter section via the illumination optical system and an optical image of the pattern is focused at the photoelectric converter section. The optical image is sent to the comparator circuit as measurement pattern data
On the other hand, design pattern data of the sample read out from the data memory is sent to the reference data generator section. The reference data generator section converts the design pattern data into pixel data. In order to correct a deviation between finish dimensions of the pattern formed on the sample and a design value, a resize and corner rounding circuit (not shown) incorporated in the reference data generator section carries out a resizing process for moving an edge position of the pattern and a corner rounding process for rounding a pattern corner portion with respect to the pixel data.
The resized and corner-rounded pixel data is subjected to a proper filtering process, the filtered pixel data is converted into an image equivalent to an optical image, and is then sent to the comparator circuit as reference pattern data. The comparator circuit compares the measurement pattern data with the reference pattern data in accordance with a proper algorithm, and determines that a pattern defect occurs if these items of data do not match.
As the above-described resizing process, there has been proposed a method which extracts a graphical feature, and carries out correction based on the extracted graphical feature (Jpn. Pat. Appln. KOKAI Publications No. 5-60699 and No. 5-198641).
However, in a mask including a fine pattern such as an optical proximity correction (OPC) pattern applied to a most-advanced mask, the minimum graphical feature of the pattern becomes smaller than the size of one pixel. Thus, in the conventional method, a graphical feature has been hardly extracted or the shape of a graphics to be processed has become very complicated, whereby the lowering of precision occurs with the resizing process.
In addition, there has been proposed a method of carrying out a resizing process by using a combination of an expansion filter and a contraction filter (Jpn. Pat. Appln. KOKAI Publication No. 2003-98117). In this method, there is a problem that an error occurs with an optical filter at a later stage because a pattern edge blurs due to the filtering process.
If the precision of the resizing process is lowered as described above, the coincidence between the measurement pattern data and the reference pattern data is lowered. A false defect occurs because a defect free region is determined to be defective. This false defect causes the lowering of the defect detection sensitivity.